- Addition, subtraction, multiplication of powers are covered, see
  src/rules/powers for more details.
- All cases have been unit tested.
- Match functions have been added to the rules list in comments, to be
  uncommented when verified.

All unit tests are converted to the new tree representation.

match_combine_polynomes will return (n0, n1, c0, c1, r0, e0), thus avoiding the
duplicated arguments. Adjusted the unit tests as well.

Introduced new symbols "?" (suggest hint) and '@' (apply one suggestion).
Moved the argument parser to main.py.

Removed replace from expression{leaf,node}.

 - Node comparison using the __lt__ operator is implemented.
 - Node normalisation is implemented in extract_polynome_properties.
 - Enabled the remaining polynome test cases (all unit tests succeed).
 - Comma can be used to generate multiple expression leafs and nodes at once.